Frank ­Deruyter, of North Carolina, said: “There are a lot of people down,”

Runner Laura McLean added: “There are people who are really, really bloody.”

After the explosions, ball bearings were seen scattered around the area. There were reports shotgun shells were used in the blasts, spraying runners and spectators with shrapnel. A fire at the JFK Library five miles away was at first feared to be a another bomb.

Another device was found under the viewing gallery at the finish. That was made safe in a controlled explosion.

A suspect, said to be a 20-year-old Saudi, was last night under armed guard in hospital with shrapnel wounds.

With around 108 Irish runners in the race, there had been fears for the safety of one local woman who had been waiting at the finish to cheer her husband over the line.

After initial concerns the woman turned up safe and well.

Earlier, Eugene Coppinger, who brought 12 runners from Ireland for the race, had said: “We’ve more or less made contact with all the runners but we’re concerned about one spectator.

“She was at the finish line to cheer him on but he hasn’t been able to get in contact with her and she hasn’t reported back.”

Bobby Hillard, of Clonakilty, Co Cork, described the carnage. He said: “It was just panic.

“Everyone started running every direction, it was stressful to see a lot of runners under pressure, trying to get away even though they were literally on their last legs as they crossed the line.

“I’m feeling very fortunate. I’m one of the lucky ones and I feel very sorry for the people who were at the other end and who were 400 or 500 metres behind me but I’m feeling the best. I passed the bend and I ran up the straight which is about 400 metres and as I crossed the line, it blew.

“The first bomb happened and then the second one happened immediately after. The emergency services were on it straight away but it was complete chaos.

“It only dawned on me when I got back to the hotel and switched on the TV what had actually happened.”

Blast is seen at the right of the imageIrish runner Gareth McQuillan tweeted to say he was back in his hotel room when the explosions happened.

He revealed: “Shocking events but thankfully back in my hotel room. What a tragedy at such a great event.”

Dublin, Kerry, Louth, Cork, Kilkenny, Carlow, Galway, Westmeath, Waterford, and Cavan were among the counties listed by those who flew to Boston to take part in what is America’s oldest marathon.

Seamus Clancy, 34, from Quilty, Co Clare, who was running his second marathon in the city, also escaped injury despite crossing the finish line as the bombs went off.

His sister Michelle said the IT sales worker who lives and works in Dublin was safe.

She told the Irish Mirror: “He was in a pub about 100 metres from the finish line and only heard when it came on the TV. At first they thought it was an electrical fault.”

Gerry Carr, a Dublin-based athlete who had just finished when bombs went off, said: “It’s just pandemonium, panic, people were just running away.

“I thought it might have been a generator that had blown up, our hopes were that it was a generator that had malfunctioned.

“The time the bomb went off is the peak finish time for runners.

“People are just distraught. This is the most prestigious marathon in the world, it’s run by volunteers and people are just stunned.”

Of the runners from 74 countries who entered the marathon, more than120 were Irish.

As ­organisers prepare for next week’s London ­marathon, witnesses in Boston told of the moment the joy of finishing the gruelling event turned to terror.

One, Brian Walker, said: “It was frightening. One guy’s leg was blown off. Another of the injured was screaming, ‘My kids, I need my kids’.” Mothers were screaming for their ­children.”

Britain’s world record marathon holder Paula Radcliffe tweeted: “There are some sick people out there.”